An adverse drug reaction (abbreviated ADR) is an expression that describes the unwanted, negative consequences associated with the use of given medications. An ADR is a particular type of adverse effect. The meaning of this expression differs from the meaning of "side effect", as this last expression might also imply that the effects can be beneficial.[1] The study of ADRs is the concern of the field known as pharmacovigilance.

These interactions are usually transient and mild until a new steady state is achieved.[13][14] These are mainly for drugs without much first-pass liver metabolism. The prinicple plasma proteins for drug binding are:[15]

Patients have abnormal metabolism by cytochromeP450 due to either inheriting abnormal alleles or due to drug interactions. Tables are available to check for drug interactions due to P450 interactions.[16].